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Refractory Period 1.A second stimulus very soon after the first will not fire an AP (Absolute) 2.With a delay, a second stronger stimulus will cause a small AP (Relative) 3.With longer delay a second AP can be fired

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Absolute refractory period Relative refractory period A B C

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Why is there Refractory Period? The Na channel stays inactivated for a short period of time after it closes Inactivated Open Closed Active Closed Active